optimization of capacity and operation for cchp system...
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Optimization of capacity and operation for CCHP system by genetic algorithm
Jiang-Jiang Wang *, You-Yin Jing, Chun-Fa ZhangSchool of Energy and Power Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China
a r t i c l e i n f o
Article history:Received 11 May 2009Received in revised form 28 July 2009Accepted 1 August 2009Available online 26 August 2009
Keywords:Combined cooling heating and power(CCHP) systemOptimizationCapacityOperation strategyGenetic algorithm (GA)
a b s t r a c t
The technical, economical and environmental performances of combined cooling, heating and power(CCHP) system are closely dependent on its design and operation strategy. This paper analyzes the energyflow of CCHP system and deduces the primary energy consumption following the thermal demand ofbuilding. Three criteria, primary energy saving (PES), annual total cost saving (ATCS), and carbon dioxideemission reduction (CDER) are selected to evaluate the performance of CCHP system. Based on the energyflow of CCHP system, the capacity and operation of CCHP system are optimized by genetic algorithm (GA)so as to maximize the technical, economical and environmental benefits achieved by CCHP system incomparison to separation production system. A numerical example of gas CCHP system for a hotel build-ing in Beijing is given to ascertain the effectiveness of the optimal method. Furthermore, a sensitivityanalysis is presented in order to show how the optimal operation strategy would vary due to the changesof electricity price and gas price.
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1. Introduction
Combined cooling, heating and power (CCHP) system is broadlyidentified as an alternative for the world to meet and solve energy-related problems, such as increasing energy demands, increasingenergy cost, energy supply security, and environmental concerns[1–6]. A good CCHP system must yield economical savings, butmore importantly must yield real energy savings as well as reduc-ing the emission of pollutants. The performance of CCHP system isclosely dependent on its design and operation. Aiming to maximizethe benefits from CCHP system in comparison to traditional sepa-ration production (SP), it is necessary to optimize the design andoperation strategy.
Many studies have been reported on this topic. Better perfor-mances (e.g. operations cost, carbon dioxide emission reduction(CDER), and primary energy consumption (PEC)) can be obtainedwhen the optimization was applied to design and/or operate CCHPsystems. The optimized CCHP systems have different components.For example, the prime mover includes gas turbine [7–9], steamturbine [10,11], gas engine [12,13], a steam Rankine process usingbiomass fuels [14], and the cooling system adopts compression[15], absorption [7,15], and ejector refrigeration cycle [11], etc.
The typical optimization algorithms used in CCHP systems areusually divided to linear programming and non-linear program-ming. The linear algorithm is easily applied to CCHP system opti-mization [16–19]. The mixed integer non-linear programmingmodel is another common optimization method [9,10,14,15,20,21], which considers the non-linear characteristic and solves
the non-linear problems. There are other optimization methodssuch as sequential quadratic programming [8], tri-commodity sim-plex algorithm [17], extended power simplex algorithm [18],Lagrangian relaxation [22] and genetic algorithm (GA) [23,24].More importantly, the objective function in optimization processguides and determines the optimal result in some extent. Usually,the objective function is expressed in different terms of net cashflow, primary energy saving [25], total cost rate [8], annual totalcost [9], energy cost [7,26], exergetic efficiency and gross benefit[27], as well as carbon dioxide emissions costs [17]. Generally,the benefits achieved by CCHP system are maximized from econ-omy, energy consumption and environment.
This paper presents the general energy flow model of CCHP sys-tem and the evaluation criteria including technology, economy andenvironment. Then the objective function of the integrated perfor-mance of CCHP is constructed and GA is employed to optimize itsdesign capacity and operation. This paper is organized as follows.Section 2 presents the mathematical analysis of the CCHP systemand the evaluation criteria of CCHP systems in comparison to SP.Section 3 proposes the optimization problems and constructs theGA optimization method. Section 4 applies GA to optimize theCCHP system providing three products to a commercial buildingin Beijing, China. Some comments are concluded in the last section.
2. CCHP system
2.1. Energy flow of CCHP system
The CCHP system consists of a power generation unit (PGU),a waste recovery system, a back-up boiler, cooling system and
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* Corresponding author. Tel.: +86 312 7522443; fax: +86 312 7522440.E-mail address: [email protected] (J.-J. Wang).
Applied Energy 87 (2010) 1325–1335
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Applied Energy
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heating system, which is shown in Fig. 1. Here the cooling systemadopts the combination of electric chiller and absorption chillerbecause the excess electricity may be usually produced by CCHPsystem following thermal demand and the excess electricity isnot allowed to be sold back to grid in China. The CCHP systemoperates following thermal demand, which is a common and sim-ple operation strategy [28]. The PGU is driven by natural gas andproduces the electricity to building. The high-temperature exhaustgas of PGU is recovered to accommodate the thermal load for cool-ing in summer and heating in winter. If the heating does not com-pletely satisfy the application needs, a supplementary boiler can beused. Similarly, when the amount of generated electricity by PGU isnot enough, the additional electricity comes from the local grid. Onthe contrary, when there are excess heat or electricity produced byCCHP system, the excess energy products are dissipated from CCHPsystem. Consequently, the operation of PGU must reduce the ex-cess products when it satisfy one energy demand of building.
The balance of the electric energy in CCHP system is expressedas
Egrid þ Epgu ¼ Eþ Ep þ Eec ð1Þ
where Egrid is the electricity from grid in CCHP system (when PGUgenerates the excess electricity, Egrid is negative and its value isequal to the excess electricity. The treatment of the excess electric-ity is explained in the last assumptions of Section 2.1), Epgu is the
generated electricity by PGU, E is the electric energy use (lightsand equipments) of building, Ep is the parasitic electric energy con-sumption of CCHP system, and Eec is the electric energy consump-tion for electric chiller providing cool to building.
The electricity used by electric chiller is calculated as
Eec ¼Q ec
COPeð2Þ
where Qec is the cooling produced by the electric chiller, and COPe isthe electric chiller’s coefficient of performance (COP).
The PGU fuel energy consumption, Fpgu, can be estimated as
Fpgu ¼Epgu
geð3Þ
where ge is the PGU generation efficiency.The recovered waste heat from the prime mover, Qr, can be cal-
culated as
Qr ¼ Fpgugrecð1� geÞ ð4Þ
where grec is the heat recovery system efficiency.The heat supplied to the cooling system and heating coil is
Qr þ Qb ¼ Q rc þ Qrh ð5Þ
where Qb is the supplementary heat from the boiler, Qrc and Qrh arethe heat supplied to absorption chiller and heating coil,respectively.
The heat required by the absorption chiller and heating coil tohandle a part of cooling load and all heating load are estimatedrespectively as
Qrc ¼Q ac
COPacð6Þ
and
Qrh ¼Qh
ghð7Þ
where COPac is the absorption chiller’s COP, Qac is the cool producedby absorption chiller, Qh is heat demand for space heating anddomestic hot water, and gh is the efficiency of heating coil (hereto simplify the calculation, it is assumed that the transmission effi-ciency of domestic hot water is equal to gh).
Qrc
Egrid
Building
FmBoiler Heating
Coil
E
Qc
QhFb Qb
Qrh
Qr
HeatRecovery System
Power Generation
Unit
Fpgu
EpEpgu
Exhaust
Absorption chiller
Electricchiller
Eec
Qec
Qac
Qrc
Egrid
Building
FmBoilerBoiler Heating
CoilHeating
Coil
E
Qc
QhFb Qb
Qrh
Qr
HeatRecovery System
HeatRecovery System
Power Generation
Unit
Power Generation
Unit
Fpgu
EpEpgu
Exhaust
Absorption chiller
Electricchiller
Absorption chiller
Absorption chiller
Electricchiller
Electricchiller
Eec
Qec
Qac
Fig. 1. Energy flow diagram of CCHP system.
Nomenclature
ATCS annual total cost savingCCHP combined cooling heat and powerCDE carbon dioxide emissionCDER carbon dioxide emission reductionCOP coefficient of performanceGA genetic algorithmPGU power generation unitPES primary energy savingSP separation production
SymbolsC costE electricityF fuelN installation capacityQ heatR capital recovery factorg efficiencyl CO2 emission conversion factor
Subscriptsac absorption chillerb boilerc coole electricityec electric chillerf fuelgrid electricity gridh heatp pumppgu power generation unitr recovery heatrc the part of recovery heat for coolingrec waste heat recovery systemrh the part of recovery heat for heatingx ratio of electric cooling to cool load
SuperscriptSP separation production
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